Conventional Energy Recovering Linac (ERL)-based Free Electron Lasers (FELs) employing optical cavity resonators use a combination of off-crest acceleration and magnetic bunching to produce the high peak currents required for FEL operation. This imposes on the system a requirement for merging and separating the drive electron beam and the FEL optical mode. This is typically done with a topology as shown in attached FIG. 1 wherein the final dipole of the upstream magnetic recirculator/bunching system is the site of the final bunch length compression. Subsequent transport to a light-producing wiggler magnet 12 may (though is not necessarily will) is used to adjust transverse electron drive beam 16 properties, but does not influence bunch length. In FIG. 1, an injected electron drive beam 16 is injected into linear accelerator 18 to produce an accelerated electron beam 20.
After passage through wiggler 12, the transport system again may (though not necessarily will) be used to adjust electron drive beam 16 transverse properties, but the electron bunch remains longitudinally short until it reaches the first bending magnet 24 of the energy recovery recirculation/energy compression system of the ERL.
In this system topology, the system geometric footprint is dominated by the optical cavity length D; in the prior art, the final magnetic bending dipole 10 upstream of wiggler 12 and initial bending dipole 24 downstream of wiggler 12 are spatially adjacent to the mirrors 26 and 28 that define the optical cavity 30.
In addition to producing coherent radiation via interaction with wiggler 12 magnetic fields, the tightly bunched electron beam used in high power FELs also produces coherent synchrotron radiation (CSR) in the THz spectral regime via its interaction with the bending fields in magnetic dipoles 10 and 24. The CSR radiation from dipole 10 onward propagates down the vacuum system, and poses little operational impediment, but the radiation from dipole 24 onward propagates onto adjacent downstream mirror 28 of optical cavity 30. The resultant radiation power load can be in excess of tens or hundreds of Watts, is of asymmetric distribution on mirror 28, and is largely absorbed by many of the materials used for such mirrors. This leads to thermal distortion of mirror 28, rendering it astigmatic and consequently limiting the power that can be generated by and extracted from the FEL.
There thus remains a need to control or eliminate the radiation from dipole 24 onward that propagates onto adjacent downstream mirror 28 of optical cavity 30.